1. Field of the Invention
The present invention relates generally to a flashlight for providing a relatively constant illumination output over the useful service life of batteries employed therein despite the relatively poor voltage maintenance characteristics of the batteries during discharge. More particularly the subject invention pertains to a simple and inexpensive circuit for incorporation in a flashlight which provides a relatively constant power supply to an incandescent bulb therein over substantially the entire useful service life of the flashlight batteries. The circuit generates a pulse train for the incandescent lamp and provides a relatively constant power supply by varying the duty cycle thereof by generating pulses having a greater pulse width as the voltage from the power supply batteries declines.
2. Discussion of the Prior Art
Traditional batteries employed as a power supply for incandescent flashlights have very poor voltage maintenance characteristics over their useful service life. Traditional batteries of the kind discussed herein typically include aqueous electrolyte, manganese dioxide batteries such as zinc carbon or zinc chloride batteries or manganese alkaline batteries. As an example, a conventional zinc carbon commercially designated 908 battery provides only four to four and one half hours of continuous service, with an initial illumination output of forty-five lumens and a change in illumination over its useful life of approximately five to one, thereby producing an output near the end of its service life of only nine lumens. Moreover, as the battery discharges the lower voltage results in a substantial filament color change in the incandescent bulb from an initial white light output to a noticeably yellow light output, which is considered to be a negative factor from a consumer acceptance point of view.
Lithium based batteries now under extensive development in the art do not exhibit a four or more to one ratio decline in output voltage over their useful service life because of voltage maintenance characteristics which are vastly superior to zinc carbon, zinc chloride and manganese alkaline batteries. In view thereof, it would be desirable to provide a development such that traditionally powered flashlights remain competitive with the newly developing lithium powered lighting products.